Can't means you lack the ability to do it or that it is impossible and shouldn't is more of a suggestion as to not to do it because it is bad. In context however you would be able to tell if a can't is being more literal or if somebody just doesn't want you to do something
Using the word "can't" is a cop out. It is putting the blame on someone else. I "can't" have that ice cream before dinner, when actually they should say, I know I "shouldn't" have it. They know they are not supposed to, but they don't want to own up to their responsibility.
Don't use "can't" to mean "shouldn't". It makes you sound overly pushy. If what you really mean is "given the facts that would be a very stupid move" you can emphasize that after saying we shouldn't do something. "Can't" deserves to be used only for what it means, to indicate lack of ability.
Can't Means Lacking Ability Don't use "can't" to mean "shouldn't". It makes you sound overly pushy. If what you really mean is "given the facts that would be a very stupid move" you can emphasize that after saying we shouldn't do something. "Can't" deserves to be used only for what it means, to indicate lack of ability.
The statement above is incorrect. How you use “can’t” versus how you use “shouldn’t” depends entirely on how you are using the words in the sentence or question.
Can’t is a contraction of the word cannot. Cannot is an auxiliary verb expressing incapacity, inability, withholding permission, etc.; it is sometimes written “can not.”
You will note that the definition indicates nothing about sounding overly pushy.
When a person issues a command they want to be pushy. So when I tell my teenage child, “You can’t do whatever” it has nothing to do with that child’s ability to do it. For example, you can’t go to the movies; you can’t use my car for that long trip because the tires are too worn, etc.
“Can’t” is a more definitive word that leaves no doubt as to it meaning; it can mean that it is beyond your ability, or you don’t have permission, etc.
If I wanted to leave the decision up to whom I am talking to, I would say something like, e.g., You shouldn’t use my car for that long trip because the tires are too worn; but, if you want to assume the risk, it is your choice, or it is up to you. Here I know you have the ability to drive, but, I am letting you assume the risk of the too worn tires.
There are also many idioms where “Can’t is used and has nothing to do with ability to do something:
Beggars can’t be choosers.
You can’t teach old dog new tricks.
And, there are common expressions where can’t is used correctly and have nothing to do with ability.
Can’t you come a little earlier?
They can’t be serious?
You wouldn’t say: Shouldn’t you come a little earlier? They shouldn’t be serious? You could say it that way, but it doesn’t have as much force as using “can’t.” It loses meaning if you substitute “shouldn’t” for “can’t.”